I recollect but can't provide an exact source, that in many areas 'squatter's rights' could be claimed if overnight you built a dwelling on the common land and had a fire burning in the hearth by morning. The rights in question seem to have been the right to cultivate a bit of land, hunt, gather wild food plants, graze an animal or two and gather fuel - the same rights that other villagers had regarding common land.
When common land was seized and shared out among the wealthy in the enclosures, the poorest people simply had no voice, no vote and no come-backs. It didn't matter if they had lived in this way for generations as their living was swiped.
Many of these homes would have been made of earth 'cob' - you still get place-names like 'Cob Wall' or Cob Field'. They didn't have deep foundations and the owners had few possessions, so the sites quickly dissolved back into the earth leaving no trace.